1. Field of the Invention
The invention relates to a harmonic tone generator which is used in an audio apparatus such as an apparatus for amplifying a vocal signal from a microphone with reproduced orchestral accompaniment sound signals.
2. Description of Background Information
A construction of a conventional harmonic tone generator is shown in FIG. 1. In such a generator, an input audio signal is supplied to an HPF (high pass filter) 1. The audio signal which has passed through the HPF 1 is supplied to an amplitude limiter 2 and a signal having only components of a predetermined level or higher is derived. An output signal of the amplitude limiter 2 is supplied to a harmonics generator 3. The harmonics generator 3 comprises a clipping circuit using a nonlinear characteristic portion of a diode. The diode has input/output characteristics as shown in, e.g., FIG. 2. As compared with an input audio signal A.sub.in, an output signal A.sub.out has a waveform clipped by the non-linear characteristic portion of the diode and includes a harmonics signal. The harmonics signal which is generated from the harmonics generator 3 is attenuated by an attenuator 4 and supplied to an adder 5. The adder 5 adds the attenuated harmonics signal and the input audio signal of the original signal together, generating an addition signal. Such a harmonic tone generator with the above construction is disclosed in, for instance, Japanese Utility Model application No. 62-146313.
In the conventional harmonic tone generator, however, there is a problem such that a harmonics signal is not produced, not only in the case where the level of the input audio signal is lower than a predetermined level but also in the case where the input audio signal is of a small amplitude which doesn't reach the non-linear characteristic portion of the diode even when the level of the input audio signal is equal to or higher than the predetermined level, for example, as the relationship between an input signal B.sub.in and an output signal B.sub.out in FIG. 2.